The present invention relates to a projection exposure apparatus and exposure method which employ a photolithography process in which a pattern formed in a reticle which is a mask is exposed onto a substrate to which a photosensitive agent has been applied, when manufacturing microdevices such as semiconductor elements, liquid crystal display elements, image picking up devices (CCD), and thin film magnetic heads and the like. In particular, the present invention relates to the correction of the baseline amount, which is one of the operational amounts necessary when conducting the relative positioning of the pattern of the reticle and the substrate.
The present application is based on Japanese Patent Application No. HEI 9-361485, and the contents thereof are incorporated by reference.
In photolithography processes for manufacturing semiconductor elements or liquid crystal display elements or the like, a projection exposure apparatus is employed which obtains the semiconductor elements or liquid crystal display elements by applying a photosensitive agent onto the surface of a substrate (a semiconductor wafer surface or a liquid crystal glass substrate surface), and exposing, onto the substrate surface, via a projection optical system, the image of a reticle having formed therein a desired element circuit pattern.
As shown in FIG. 11, this projection optical apparatus 1 generally comprises a light source (not depicted in the figures) which emits illumination light for exposure which is irradiated onto a pattern formed in reticle R, a projection optical system P for reduction-projecting the pattern onto the surface of a substrate W, and a stage S for moving the substrate W below the projection optical system P, and the like.
In the photolithography process, during the above-described exposure operation, some sort of stratagem is necessary to align a plurality of shot regions on substrate W with various reticle patterns. In order to respond to this need, alignment marks (marks) A which are associated with each shot region are normally provided on substrate W, and the alignment of the reticle pattern with the regions on the substrate W, that is to say, the positioning thereof, is conducted by detecting these marks using a position detecting optical system Q which is provided separately from the projection optical system P.
As shown in FIG. 11, this position detecting optical system Q is an off-axis optical system having an optical axis QX which is parallel to the optical axis PX of the projection optical system P described above, and comprises an illumination optical part Q1 which irradiates a broadband light (having a wavelength within a range of approximately 550 to 750 nm) onto the alignment mark A, an imaging optical part Q2 into which is inputted the light generated by the illumination of the alignment mark A, and which forms an image of the alignment mark A on image pickup element Qc, and image processing part Q3 which is connected to the image pickup element Qc. In imaging optical part Q2, an index plate Qk which is provided with index marks is provided in the optical path, and an image of the index marks of this index plate Qk is formed on the image pickup element Qc. The image processing part Q3 detects the amount of positional displacement between the image of the index marks formed on the image pickup element Qc and the image of the alignment marks A. An alignment controller (not depicted in the figure) conducts positioning by moving the stage S based on this amount of positional displacement and the position of the stage S which is detected by a laser interferometer during the image pickup of alignment marks A.
In alignment such as that described above, an operational amount, termed a baseline amount, is generally required, and this is obtained in the manner described below. Now, the reference mark FM which is formed on the stage S is detected by the positional detection optical system Q. At this time, the amount of positional displacement with the image of the index marks on index plate Qk is detected, and the position of stage S during the detection of the reference mark FM is determined. Furthermore, based on the amount of positional displacement and the position of stage S, the position X1 of stage S when the amount of positional displacement is 0 is determined. This position X1 is stored in the storage region of an alignment controller, which is not depicted in the figure, of projection exposure apparatus 1.
Next, the stage S is moved so that the reference mark FM is essentially directly beneath the projection optical system P, or in other words, so that the reference mark FM is disposed at a position conjugate with the reticle marks Rm via projection optical system P. An image of the reticle marks Rm and an image of the reference mark FM projected by the projection optical system P are formed on the image pickup element of an alignment optical system (not depicted in the figure) which is disposed above the reticle R. Additionally, the reticle marks Rm form a reference during alignment. The alignment optical system detects the amount of positional displacement of these two mark images. The alignment controller determines the position X2 of the stage S when the amount of positional displacement is 0, based on the amount of positional displacement and the position of stage S as determined by a laser interferometer. This position X2 which is thus determined is stored in the storage region of the alignment controller described above.
The baseline amount B is obtained using position X1 and position X2, which relate to the stage and which were determined as described above; the baseline amount B is equal to X2xe2x88x92X1.
When the image of the reticle pattern is transferred onto a shot region on substrate W, the alignment mark A belonging to this shot region is detected by the positional detection optical system Q and the position thereof is determined, and the stage S is moved based on this determined position and the baseline amount B described above. By means of this, the image of the reticle pattern Rm is accurately aligned with the shot region.
In this way, the baseline amount B is an operational amount which is extremely important in the photolithography process, and a strictly accurate measured value thereof is required. However, here, there are a number of difficult problems which need to be solved.
For example, when semiconductor elements are manufactured, a number of types of semiconductor wafers having different reflectances and the like are employed, and the thin films or optical characteristics of the plurality of layers layered on these semiconductor wafers also differ. Furthermore, the alignment marks which are formed on these layers together with the circuit patterns themselves may change in shape in the process of etching and the like. Accordingly, it is difficult to always precisely detect the position of the alignment marks using the same positional detecting optical system irrespective of the type of semiconductor wafer or layer or the like.
In order to take account of this state of affairs, there have been proposals to increase the detection accuracy of the alignment mark image by improving the positional detection optical system Q, as disclosed in Japanese Patent Application, First publication, No. HEI 8-327318 and the corresponding U.S. Pat. No. 5,706,091. The direct problem to be solved in the above application relates to the fact that, as a result of the flattening process of the semiconductor wafers, changes in the unevenness of the alignment mark provided on the wafer become extremely small, and as a result, the detection of the mark becomes difficult.
In order to solve this problem, as shown in FIG. 11, the invention disclosed in the documents described above is provided, in the illumination optical part Q1 and the imaging optical part Q2 of the positional detection optical system Q, with an illumination light limiting member q1 and a phase plate q2 which may be inserted into or withdrawn from the optical path, and by changing the optical characteristics within the positional detection optical system Q, the detection of the image in image pickup element Qc is conducted in a satisfactory manner.
In a positional detection optical system Q such as that described above, it is possible to arrive at the detection of the alignment mark in a satisfactory and certain manner. However, in this case, by disposing the illumination light limiting member q1 and the phase plate q2 on the optical axis, the optical characteristics of the positional detection optical system Q are changed, and this has an effect on the accuracy of alignment. Here, concrete examples of modifications in the optical characteristics include those in which shifts are produced in the optical axis. That is to say, an offset (detection error) is produced in the measured value of the positional detection optical system Q. Now, if the baseline amount when members q1 and q2 are not inserted into the optical path is represented by B, then between this amount B and a baseline amount Bxe2x80x2 which results when the members are inserted into the optical axis, an amount of displacement is produced such that Bxe2x80x2=B+xcex94B (where xcex94B does not equal 0). Accordingly, even if the substrate W is moved in accordance with a baseline amount B measured prior to the insertion or removal of the members q1 and q2, it is not possible to accurately align the shot region on the substrate W with the reticle pattern.
Furthermore, an amount of displacement related to the baseline amount, such as the xcex94B described above, may be produced even by changes in the characteristics of the electrical circuits in the image processing system Q3. For example, in this image processing system Q3, a signal amplifier is normally disposed; however, changes in the amplification factor of this signal amplifier may produce an amount of displacement xcex94b. By means of this, the baseline amount B which was previously measured, receives a displacement, and the value thereof becomes Bxe2x80x3, which equals B+xcex94b. Accordingly, by means of this, as well, it becomes impossible to accurately align the shot region on the substrate W with the reticle pattern.
The present invention was created in light of the above circumstances; it has as an object to provide a projection exposure apparatus which makes it possible, even where the characteristics of the positional detection optical system have changed, to correct the baseline amount in correspondence with this change in characteristics, and to accurately conduct alignment. Furthermore, it has as object of the present invention to provide an exposure method which permits the overlapping transfer of reticle patterns onto circuit patterns (shot regions) formed on a substrate, with constant good accuracy, even when the mark detection conditions of the positional detection optical system change.
In order to attain the above object, the projection exposure apparatus in accordance with the present invention is a projection exposure apparatus in which marks on a substrate are detected by a positional detection optical system, the positional relationships between a substrate and a mask are adjusted based on a baseline amount and the results of this detection, and an image of a pattern on the mask is projection-exposed onto the substrate by a projection optical system. This apparatus comprises a first modification device for modifying the optical characteristics of said positional detection optical system, and a baseline correction device for correcting the baseline amount in accordance with the modification of the optical characteristics by the first modification device.
In accordance with this apparatus, each time the optical characteristics of the position optical system are modified by the first modification device, the correction of the baseline amount is conducted by the baseline correction device. That is to say, with respect to the optical characteristics of the positional detection optical system in all the variously modified states which are contemplated, the proper corresponding baseline amount is applied, and it is possible to execute reliable alignment. By means of this, it is possible to provide products of higher quality in semiconductor elements and the like which are manufactured by a photolithography process.
In another aspect of the present invention, a signal processing system, which is connected to the positional detection optical system, and a second modification device, which modifies the electrical circuit characteristics of the signal processing system, are further provided and the baseline correction device corrects the baseline amount in accordance with at least one of the modification of the optical characteristics and the modification of the electrical circuit characteristics.
Another aspect of the present invention is a projection exposure apparatus in which a mark on a substrate is detected by a positional detection optical system, and after aligning the position of the substrate and a mask based on the baseline amount from the detection results, the image of a pattern of the mask is projection-exposed onto the substrate by a projection optical system. This projection exposure apparatus comprises a second modification device which modifies the electrical circuit characteristics of the positional detection optical system, and a baseline correction mechanism which corrects the baseline amount in accordance with the modification of the electrical circuit characteristics by the second modification device.
In accordance with this aspect, each time the electrical circuit characteristics of the position optical system are modified by the second modification device, the correction of the baseline amount is conducted by the baseline correction mechanism. That is to say, with respect to the electrical circuit characteristics of the positional detection optical system, a variety of modified states of which may be contemplated, the proper corresponding baseline amount is applied, and it is possible to conduct reliable alignment.
Another aspect of the present invention is characterized in that at least one of a phase plate and a partial shielding plate which may be inserted and retracted with respect to the optical axis of the positional detection optical system is employed as the first modification device.
In accordance with this aspect, as a result of the modification of the optical characteristics by at least one of the phase plate and the partial shielding plate, it is possible to increase the accuracy of detection of the mark. By means of this, the optical characteristics of the positional detection optical system also receive modification, and the baseline amount is modified in comparison with the state prior to insertion of the phase plate and the partial shielding plate; however, this is corrected by the baseline correction mechanism. Accordingly, it is possible to execute reliable alignment.
The second modification device may modify the amplification factor of the mark detection signal. In accordance with this, by means of the modification of the amplification factor of the mark detection signal, it is possible to increase the accuracy of detection of the mark. In this case, the electrical circuit characteristics of the positional detection optical system also undergo modification, and the baseline amount is modified in comparison with the state prior to undergoing amplification; however, this is corrected by the baseline correction mechanism.
The baseline correction mechanism may measure the baseline amount after modification based on the modification of the optical characteristics conducted by the first modification device or based on the modification of the electrical circuit characteristics conducted by the second modification device. In this case, in the modification of each set of characteristics, it is possible to first obtain the accurate baseline amount for each characteristic modification from a measurement of the baseline amount corresponding to the modification, and thus to conduct alignment. By means of this, when the baseline amount is measured for different characteristic modifications, it is possible to conduct accurate exposure operations.
The baseline correction mechanism may comprises a storage unit for storing in advance the correctional values for each baseline amount based on the modification of the optical characteristics conducted by the first modification device or based on the modification of the electrical circuit characteristics conducted by the second modification device, and a control unit which detects the modification conducted by the first modification device or the modification conducted by the second modification device and which obtains the correction value corresponding to the modification from the storage unit.
In this case, a plurality of baseline correction values corresponding to the various conceivable modifications of characteristics are prepared together in advance, and thereby, during alignment, the most appropriate baseline correction value is selected from among these, and then alignment is conducted. Accordingly, it is not necessary to individually measure the baseline amount for each modification of characteristics, and it becomes possible to rapidly conduct operations. Accordingly, it is not merely possible to realize highly accurate exposure operations, but simultaneously to increase throughput.
The exposure method in accordance with the present invention is a method in which a mark formed on a substrate is detected by a mark detection system, the relative position of an exposure beam emitted from a mask and the substrate is adjusted based on the results of this detection, and the substrate is exposed by this exposure beam. This method comprises modifying the detection conditions of the mark on the substrate by the mark detection system, and based on the detection errors of the mark detection system produced in accordance with the modification of the detection conditions, performing adjustment of the relative positions of the exposure beam and the substrate after modification of the detection conditions.
In accordance with this method, even after modification of the mark detection conditions in the mark detection system, adjustment of the relative positions of the exposure beam and the substrate is conducted based on the detection errors generated in accordance with the modification of the detection conditions, so that an image of the pattern of the mask can be projected onto the appropriate shot region. Accordingly, exposure onto the substrate is conducted accurately, and it is possible to obtain high quality products.
It is also possible to modify the detection conditions by modifying at least one of the intensity distribution of the illumination light, which is irradiated onto the mark on the substrate within the mark detection system, and the imaging characteristics of the light generated from the mark by the irradiation of the illumination light. In this case, what is meant by the modification of the detection conditions is modification of at least one of the intensity distribution of the illumination light irradiated onto the mark and the imaging characteristics of the light emitted from the mark, and by appropriately adjusting these, it is possible to increase the accuracy of detection of the mark. Mechanisms such as, for example, orbicular zone illumination, total illumination, or the like are contemplated for the modification of the intensity distribution of the illumination light. The meaning of this is that the state of the reflected light or diffracted light emitted from the mark is modified. Furthermore, methods such as, for example, the production of phase difference at a certain position in the cross section of the beam are contemplated for the modification of the imaging characteristics of the light emitted from the mark.
In order to compensate for fluctuations in the baseline amount of the mark detection system as a result of errors in detection, the baseline amount of the mark detection system which is employed in the adjustment of the relative positions of the baseline amount exposure beam and the substrate may be corrected in accordance with the detection conditions after modification.
Including the detection errors produced as a result of modifications in the detection conditions, the baseline amounts of the mark detection system are calculated in advance for each set of detection conditions, and it thus becomes possible to execute the adjustment of the relative position based on the baseline amounts corresponding to the detection conditions after the modification of the detection conditions.
In this case, after the inclusion of the detection errors generated as a result of the modification of the detection conditions, the individual baseline amounts corresponding thereto are determined in advance, and the baseline amounts corresponding to the individual modifications of detection conditions are applied in adjusting the relative positions of the exposure beam and the substrate, so that alignment may be executed rapidly without delays. Accordingly, not only are highly accurate exposure operations realized, but there is also the prospect that throughput will simultaneously increase.